FLASH FLOOD PREDICTION James McDonald 4/29/08. Introduction - Relevance 90% of all national disasters are weather and flood related Central Texas.

FLASH FLOOD PREDICTION

James McDonald 4/29/08

Introduction - Relevance

90% of all national disasters are weather and flood related

Central Texas – “Flash Flood Alley”

Project Goal – To compare flash flood prediction on multiple scales (NWS, LCRA, City of Austin)

Agencies – The main players NWS – ultimately responsible for issuing flood

warning LCRA – manages flood releases from the Highland

Lakes City of Austin – flood monitoring and emergency

management

NWS – West Gulf River Forecasting Center

1 of 13 NWS River Forecasting Centers

Also includes in house Meteorological forecasting

NWS River Forecast Procedure

NexRAD Radar

4 km x 4 km grid cell Rain gages across state used to calibrate

radar Radar - mean areal precipitation (per

hour)

Sub-basins

Delineated by USGS streamgage

Total QPF (Quantitative Precip. Forecast) 6 hr. UH time step Stage height by

USGS rating curve

NWSRFS - (NWS River Forecast System)

Similar to HEC-HMS (good for design)

NWSRFS – good for real time flood forecasting

Sacramento Soil Moisture Accounting Method (SAC-SMA) Able to adjust soil

moisture to tweak model

Human Adjustment

Distributive Modeling

The Future Finer rainfall distribution instead of mean

areal across entire sub-basin Rainfall variation - GIS

LCRA - Lower Colorado River Authority

The Forecast Spreadsheet – early 1980’s

Inputs: Colorado River

Forecast @ San Saba

Time-lagged tributary hydrographs

WSEL of Lake Buchanan & Lake Travis

Reservoir releases

The Spreadsheet - Forecasting

Catchment Forecast System (CFS) Post 1997 flood Integrates HEC

programs: PRECIP HEC1 UNET

Hydromet: Hydrometeorological Data Acquisition System

HEC’s Corps Water Management System (CWMS)

Integrates the following models: HEC-HMS, Hydrologic

Modeling System 289 sub-basins 205 routing reaches 18,340 mi^2

HEC-ResSim, Reservoir Evaluation System Simulation 7 reservoirs

HEC-RAS, River Analysis System 864 cross-sections 347 river miles

History of Austin flooding

November 15, 2001: 8” of rain in 6 hr. (reported up to 15”)October 17, 1998: 31 deaths 7,000 people evacuated from homesProperty damages ~ $1 billion454 Austin homes damaged

December 20, 1991: 200 homes in Travis and Bastrop counties were completely under water

May 24, 1981: 13 drowned, $36 million in damages, Shoal Creek 90 gpm -> 6 million gpm

City of Austin – Am I flooding now? Flood Early Warning System Emergency Operations Center

Austin Watersheds – Time is not our friend

7,000 homes in Austin floodplain

Tc range 30min. – 6 hr.

Lowering arm – low water crossing – marginal success

20 roads close with 5-yr storm

Flood Early Warning System

Components include: ~40 creek and lake gauges ~80 rain gauges Automated data transfer

every 15 minutes to the National Weather Service

High Hazard Dam Action Plan

Conclusions – Final Comparisons

Special thanks to:

Mike Shultz, National Weather Service, Ft. Worth, TX

Matt Ables & Melinda Luna, LCRA Susan Janek, City of Austin, FEWS

Questions?